1. Technical Field of the Invention
The present invention relates to an apparatus for detecting a charged state (i.e., “internal charged state” or simply, “internal state”) of a secondary battery, which is able to reduce influences of battery polarization on estimation of a charged state of an electric storage device for vehicles.
2. Related Art
With the advance of larger capacity electric storage devices for vehicle and with the increasing growth in size of on-vehicle electric loading devices, accuracy has been demanded in measuring a charged state, such as an SOC (state of charge) indicative of a charged state of a battery or an SOH (state of health) Indicative of deterioration in the charged state of a battery, in an electric storage device for vehicles, so that overcharge or over-discharge can be prevented.
Known SOC detecting methods include a charge/discharge current integrating method, and a method for estimating the SOC based on battery characteristics (hereinafter also referred to as a “battery characteristics computing method”). The “battery characteristics computing method” herein refers to a method for estimating the SOC based on a relation of a voltage V and a current In of a battery with respect to the SOC. In particular, a pseudo open-circuit voltage Vo of a battery is known to have a strong correlation with the SOC, and hence a method is known in which the SOC is calculated based on the pseudo open-current voltage Vo which is calculated based on a number of measured voltages V and currents I.
Japanese Patent Application Laid-Open No. 9-297163 indicates one example of such a battery characteristics computing method and suggests an SOC calculating method in which a characteristics line indicative of voltage/current characteristics is determined using a least-squares method based on a number of paired data of current and voltage (also referred to as “data pair” or “voltage/current pair”) detected from a battery, a maximum dischargeable power is calculated based on the characteristics line, and an SOC is determined based on the maximum dischargeable power.
However, in various types of conventional SOC computing methods, a problem has been raised that large errors are caused by battery polarization. For example, in case where charge/discharge polarization effect is so large as to have the relation between the battery voltage and the SOC largely changed depending on the charge/discharge history, such as a case of a lead-acid battery, an SOC estimation accuracy is significantly deteriorated. Therefore, in the charge/discharge current integrating method, auxiliary charge periods are required for periodically determining a specific SOC. This, however, has raised another problem that the fuel consumption is deteriorated. It has been a practice, therefore, to reduce a normally used SOC region of a battery in consideration of the variation in an amount of polarization to prevent overcharge or over-discharge. This, however, has raised a further problem that the weight and volume of the battery for realizing a required amount of charge/discharge is unavoidably increased.